1. Field of the Invention
The invention relates generally to a process for improving the properties of raw or "green" cokes obtained by known processes from materials of petroleum origin and particularly to a process for calcining and desulfurizing such cokes to provide a product having acceptable sulfur content with satisfactory density characteristics.
Industrial petroleum coke is manufactured by methods well known in the art, the major source being the delayed coker. Unfortunately, many petroleum cokes produced by this method and other known methods contain appreciable amounts of sulfur, and cannot be directly utilized in the fabrication of some carbon products due to this impurity. Aluminum producers, for example, the largest consumer in total quantity of calcined petroleum coke, require low sulfur coke to satisfy environmental regulations. These producers currently specify that the sulfur content of these cokes must be at a level of no more than about 2.5 wt.% to be acceptable for use in the fabrication of anodes for aluminum reduction cells.
Raw petroleum coke for industrial purposes is conventionally calcined at temperatures in the range of about 1150.degree.-1300.degree. C. by methods well known in the art to remove substantially all of the volatile matter content of the coke and to provide increased density and conductivity therefor. It is known that the customary methods utilized for petroleum coke calcination are, in and of themselves, not adequate to bring about desulfurization of the coke without deterioration of other important coke properties.
A physical property of calcined petroleum coke recently recognized by those in the art as useful in predicting the apparent density, strength, and consumption rate of baked carbon anodes made from that coke in aluminum (Hall) cells is vibrated bulk density (VBD). A method for determining this property generally comprises placing a 100.0 gram sample of the calcined coke particles sized between 300 and 850 microns (-20/+48 mesh Tyler Screen Scale) in a 250 cc graduated cylinder mounted in a jogger (shaker) unit and vibrating the cylinder for 5 minutes at a predetermined jogging rate at which maximum particle compaction occurs. The volume of the compacted coke particles is recorded and the VBD, expressed in g/100 cc, is calculated as follows: EQU VBD=(A/B).times.100
where:
A=sample weight in grams PA1 B=compacted volume in cubic centimeters.
The particle size of the coke sample used in the VBD determination is approximately midpoint in the conventional anode aggregate particle size distribution.
It has been found that a VBD value for calcined coke of at least about 78 g/100 cc is necessary to provide acceptable quality for use in anode production.
2. Description of the Prior Art
It is known in the art that the temperatures at which calcination of high sulfur raw petroleum coke is conventionally carried out are not sufficient to reduce the coke's sulfur level to a value acceptable to consumers.
One method known for desulfurizing raw coke comprises directly heating the coke in a single stage to a temperature above about 1500.degree. C. in a rotary kiln or the like. Experience has taught that while this procedure effectively reduces the coke's sulfur content, the VBD and other physical properties are substantially deteriorated during the heat treatment process, as compared to coke properties after calcination at conventional temperatures.
U.S. Pat. No. 4,160,814 to Hardin et al. provides a two stage process for calcining and thermally desulfurizing raw petroleum coke without lowering its bulk density (BD), as defined below, comprising heating the coke at 490.degree. C. to 850.degree. C. for 30 to 60 minutes while retaining at least 30 wt. % of the coke's volatile matter content, then heating the partially devolatilized coke at a temperature of at least 1500.degree. C. for 30 to 70 minutes to calcine and desulfurize the coke. The BD value referred to in the patent is the weight per unit volume of the coke particles, and is determined by transferring a weighed sample of the coke, having a particle size either in a range of 3.36 to 4.76 mm (-4/+6 mesh Tyler Screen Scale) or Run of Kiln (ROK) size, into a graduated container and calculating the BD from the displaced volume and sample weight. While the process provided in the 4,160,814 patent advanced the art of coke desulfurization over known processes by providing retention of normal bulk density values, it was learned that the coke product exhibited lowered VBD properties compared to conventionally calcined coke, indicating decreased strength and increased consumption of anodes made from coke produced according to this patent, compared to coke calcined by conventional methods without desulfurization.